Engines may be configured with various fuel systems used to deliver a desired amount of fuel to an engine for combustion. One type of fuel system includes a port fuel injector and a direct fuel injector for each engine cylinder. The port fuel injectors may be operated to improve fuel vaporization and reduce engine emissions, as well as to reduce pumping losses and fuel consumption at low loads. The direct fuel injectors may be operated during higher load conditions to improve engine performance and fuel consumption. Additionally, both port fuel injectors and direct fuel injectors may be operated together under some conditions to leverage advantages of both types of fuel delivery.
As such, there may be operating conditions where engines configured with dual fuel injection capabilities operate for an extended period with one of the injection systems inactive. For example, there may be conditions where the engine is operated with port injection only and the direct injectors are maintained inactive. The direct injectors may be coupled to a high-pressure fuel rail downstream of a high-pressure fuel pump. During the extended periods of non-operation of the direct injectors, the presence of a one-way check valve may result in high-pressure fuel being trapped in the high-pressure fuel rail. If the stagnating fuel is exposed to higher temperatures (such as higher ambient temperatures), the fuel may begin to expand and vaporize in the fuel rail, resulting in an increased fuel pressure, due to the closed and rigid nature of the fuel rail. This increased fuel temperature and pressure may in turn affect the durability of both the direct fuel injectors and related fuel hardware, in particular when the direct fuel injection system is enabled again.
One example attempt to address direct fuel injector degradation due to elevated fuel rail pressure includes activating an alternate injector in response to a fuel rail temperature increase. For example, in the approach shown by Rumpsa et al. in U.S. 2014/0290597, when operating an engine cylinder with fuel from a port fuel injector and not a direct injector, the direct injector is activated in response to a temperature increase of a direct injection fuel rail to above a threshold. The flow through the direct injector is activated for a predetermined amount of time, which in some examples is based on a predetermined injection mass.
However, the inventors herein have recognized potential problems with such an approach. As an example, activating the direct injector for a predetermined amount of time may result in fueling errors. Specifically, fuel rail pressure may fall below a minimum desired direct injection pressure, thereby resulting in unpredictable fuel injection masses. The fuel metering error may result in torque errors as well as undesirable exhaust soot emissions. Additionally, increasing the fuel rail pressure in response to the pressure falling below a minimum desired direct injection pressure may result in increased NVH and reduced energy efficiency, both of which are undesirable for a vehicle operator. Still further, injecting a predetermined amount of fuel (e.g., injecting for a predetermined amount of time or directly injecting a predetermined fuel mass) may include injecting with a large proportion of direct injection to port fuel injection, thereby resulting in degraded engine performance.
In one example, the issues described above may be addressed by a method comprising: while operating an engine cylinder with fuel from only a first injector, transiently activating the second injector to inject fuel into the cylinder in response to a fuel pressure increase at a fuel rail coupled to the second injector, and deactivating the second injector in response to a fuel pressure decrease at the fuel rail below a lower threshold, the lower threshold adjusted based on one or more engine operating conditions.
As one example, during conditions when an engine is operated with port injection only, a direct injector may be intermittently activated and deactivated to maintain a fuel pressure within a desired range. Specifically, while maintaining a high-pressure fuel pump disabled, engine direct injectors may be selectively activated when fuel pressure in the high-pressure direct injection fuel rail reaches an upper threshold. Fuel may be injected from the direct injectors until the fuel rail pressure reaches a lower threshold. Further, the lower threshold may be adjusted based on operating conditions while maintaining the lower threshold above a level where the high-pressure fuel pump needs to be re-enabled. For example, the lower threshold may be increased when engine operating conditions indicate that port fuel injection is preferred for engine performance, such as during cold ambient conditions, or when the exhaust soot load is already elevated. When the lower threshold is increased, relatively less fuel may be delivered from the direct injectors to the cylinder, while relatively more fuel may be delivered via the port injectors to the cylinder. Alternatively, the lower threshold may be decreased when engine operating conditions indicate that at least some direct injection is desired, such as when an engine's propensity for pre-ignition is higher, or when the alcohol content of the injected fuel is higher. When the lower threshold is decreased, relatively more fuel may be delivered from the direct injectors to the cylinder, while relatively less fuel may be delivered via the port injectors to the cylinder. In this way, direct injector degradation may be reduced, while still maintaining a desired level of engine performance achieved by delivering fuel to the engine via port injectors.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.